The present invention relates to apparatus and methods for monitoring cell voltages in fuel cell stacks. In particular, the present invention relates to monitoring cell voltages for purposes of detecting and preventing voltage reversal conditions in solid polymer electrolyte fuel cells.
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits. To be commercially viable however, fuel cell systems need to exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside the preferred operating range.
Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in solid polymer electrolyte fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are placed at each end of the stack to hold it together and to compress the stack components together. Compressive force is needed for effecting seals and making adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
However, fuel cells in series are potentially subject to voltage reversal, a situation where a cell is forced to the opposite polarity by the other cells in the series. This can occur when a cell is unable to produce the current forced through it by the rest of the cells. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the fuel cell.
The adverse effects of voltage reversal can be prevented, for instance, by employing rectifiers capable of carrying the stack current across each individual fuel cell. Alternatively, the voltage of each individual fuel cell may be monitored and an affected stack shut down if a low cell voltage is detected. However, given that stacks typically employ numerous fuel cells, such approaches can be quite complex and expensive to implement.
The complexity and expense of cell voltage monitors can be reduced by monitoring and comparing the voltages of suitable groups of cells instead of individual cells, as disclosed in U.S. Pat. No. 5,170,124. Another approach is disclosed in European Patent publication number EP 982788 in which optoisolators are used to monitor voltages across pairs of cells in a stack. A photoemitter activates and illuminates a photo-detector in each optoisolator when the summed voltage of the monitored pair of cells exceeds an activating voltage. The photo-detectors are connected in series and current can pass through when all the photoemitters are activated. However, a low voltage condition in any pair causes the associated photoemitter to darken, opening the photo-detector, and interrupting the flow of current. The absence of current flow is detected and provides warning of potential voltage reversal conditions in the stack. There are certain disadvantages with using optoisolators however. For instance, optoisolators are yes-no devices and do not provide an indication of the severity of the problem. Further, the response of an optoisolator may be fast enough that noise spikes might affect output, thereby necessitating signal conditioning. Most importantly, optoisolators have a specific xe2x80x9conxe2x80x9d voltage (about 1.2 V) which can limit their effectiveness in this application. For instance, in a stack where the normal operating cell voltage was always over 0.6 V, a pair of xe2x80x9cnormalxe2x80x9d cells would reliably activate the optoisolator. However, if the normal operating cell voltage could be about 0.6 V or less, then a pair of xe2x80x9cnormalxe2x80x9d cells would not reliably activate the optoisolator. In addition, monitoring a three cell group could not be an acceptable option in some circumstances because a three cell group consisting of two xe2x80x9cnormalxe2x80x9d cells operating at 0.8 V and one cell undergoing voltage reversal could still activate the optoisolator.
Other approaches may also be considered in order to detect voltage reversal conditions. For instance, a specially constructed sensor cell may be employed that is more sensitive than other fuel cells in the stack to certain conditions leading to voltage reversal (for example, fuel starvation of the stack). Thus, instead of monitoring many cells in a stack, only the sensor cell need be monitored and used to prevent widespread cell voltage reversal under such conditions. However, other conditions leading to voltage reversal may exist that a sensor cell cannot detect (for example, a defective individual cell in the stack). As another example, an exhaust gas monitor may be employed that detects voltage reversal by detecting the presence of or abnormal amounts of species in an exhaust gas of a fuel cell stack that originate from reactions that occur during reversal. While exhaust gas monitors can detect a reversal condition occurring within any cell in a stack and they may suggest the cause of reversal, such monitors do not generally provide any warning of an impending voltage reversal. Thus, voltage monitors offer certain advantages over these other approaches and may therefore be desirable for voltage reversal protection.
An improved cell voltage monitor for fuel cell stacks employs voltage monitoring units for monitoring cell voltage. The voltage monitoring units comprise a heating resistor, a rectifier in series with the heating resistor, and a sensing thermistor. A thermistor is a thermally sensitive resistor whose resistance varies predictably as a function of temperature. The sensing thermistor is used to sense heat generated by the heating resistor and, for this purpose, is positioned to be in thermal communication with, but electrically isolated from, the heating resistor.
The series-connected heating resistor and rectifier in the voltage monitoring unit are electrically-connected across one or more fuel cells in the stack. The polarity of the rectifier is arranged such that current is prevented from flowing through the series-connected resistor and rectifier when the voltage across the fuel cell or cells changes polarity during voltage reversal. During normal operation of the fuel cell stack however, the heating resistor is resistively heated by the fuel cell. The amount of heat generated and communicated to the sensing thermistor is a function of the voltage across the heating resistor. Thus, the temperature and hence the resistance of the sensing thermistor are indicative of fuel cell voltage.
To detect a low voltage condition, a determination is made as to whether the sensing thermistor resistance is in a range associated with low cell voltage. In this regard, a signal indicative of the sensing thermistor resistance can be obtained using a reference thermistor. For instance, the reference thermistor may be connected in series with the sensing thermistor but is thermally isolated therefrom. A reference voltage may then be applied across the series-connected sensing and reference thermistors and the voltage across the sensing thermistor measured. The resistance of the sensing thermistor may then be determined by comparing the voltage across the sensing thermistor to the reference voltage. An advantage of this approach is that the comparison can provide information about the extent of a low voltage condition along with the mere indication of the problem.
A preferred embodiment employs reference and sensing thermistors having the same resistance versus temperature characteristics (for example, same type). A diode may also be employed to prevent other signals (for example, signals from other voltage monitoring units) from affecting the sensing thermistor voltage. One end of the diode may be electrically-connected between the series-connected sensing and reference thermistors.
Resistance changes in the sensing thermistor are more readily detected if the resistance changes substantially with an increase in temperature above ambient. A resettable thermal switch, such as a polymeric positive temperature coefficient (PTC) fuse, may undergo a marked increase in resistance with temperature and is suitable for use as a sensing thermistor. In such an embodiment, the PTC fuse might be heated sufficiently during normal operation to be xe2x80x9copenxe2x80x9d, but insufficiently heated during a low voltage condition and be xe2x80x9cclosedxe2x80x9d. A low voltage condition might thus simply be indicated by determining if the PTC fuse was open or closed. However, this approach does not provide quantitative information about the extent of a low voltage condition.
Preferably, the heating resistor in the voltage monitoring unit is also a positive temperature coefficient thermistor (that is, a heating thermistor) to reduce power consumption over the higher end of the normal operating voltage range of the fuel cells in the stack.
A cell voltage monitor for a fuel cell stack may comprise a plurality of the preceding voltage monitoring units. The series-connected heating resistors and rectifiers in each of the plurality of voltage monitoring units may themselves be connected in series. The plurality of voltage monitoring units may conveniently be mounted on a flexible printed circuit board. The heating resistor, the rectifier, and the sensing thermistor in a given voltage monitoring unit may be mounted closely together and coated with a suitable thermally conductive material, such as a thermosetting polymer. Other components may be mounted at a distance for purposes of thermal isolation.
In an embodiment employing reference thermistors and diodes (such as that described above), the series-connected reference thermistors and sensing thermistors in each of the plurality of voltage monitoring units are conveniently connected in parallel across the reference power supply. The other ends of the diodes may be connected to a common signal line and the polarity of the diodes may be set such that current is prevented from flowing from an unheated sensing thermistor to any heated thermistor. In this way, when the sensing circuit is arranged such that an unheated sensing thermistor generates a greater signal than the heated ones, the diodes stop the greater signal from sinking through the heated thermistor circuits. The presence of the greater signal on the signal line is indicative of a low voltage cell.
In an embodiment employing resettable thermal fuses (such as that described above), the resettable thermal switches in each of the plurality of voltage monitoring units may be connected in parallel. An xe2x80x9copenxe2x80x9d circuit condition might then exist in this parallel circuit of switches when all the cells are operating in a normal voltage range. However, a single low voltage cell would result in a xe2x80x9cclosedxe2x80x9d circuit condition in one of the fuses and hence in the parallel circuit.
A voltage monitoring unit may be used to monitor the voltage of one or more fuel cells (for example, a pair of cells) in a fuel cell stack by electrically-connecting the series-connected heating resistor and rectifier in the unit across the cell or cells. In order to monitor the voltages of all the series-connected fuel cells in a fuel cell stack, generally a cell voltage monitor would be employed which comprises a plurality of voltage monitoring units. It is desirable to thermally isolate the sensing thermistors in the voltage monitoring units from the fuel cell stack itself in order to prevent heat generated by the stack from interfering with the function of the voltage monitoring units.
The aforementioned voltage monitoring units and cell voltage monitor are useful for monitoring for voltage reversal in fuel cell stacks, especially solid polymer electrolyte fuel cell stacks. Desirably, a low voltage in any fuel cell in the stack may be detected and an appropriate action may then be initiated to prevent damage to the cells.